An air conditioner for a vehicle which has a first chamber having a heat exchanger, a second chamber having at least one ventilation diffuser and a third chamber having heater and defroster diffusers, said third chamber being connected in the downstream of said second chamber, and a switching damper provided to control the communication between said second chamber and said third chamber. The air conditioner has a remarkably small air resistance and can obtain sufficient air flow rate in the ventilating made both dynamic pressure or other forcible types and has the degree of freedom necessary for designing a predetermined space in the lower portion of the chamber having the ventilation diffusers.